Trane RAUC-IOM-15, Installation & Maintenance Instructions Manual

Page: 49 / 68

49
Sequence of Operation
VAV W7100A Discharge Air Controller (7U11)
The discharge air controller used in Variable Air Volume ap-
plications is a Honeywell W7100A. This microprocessor
controller is designed to maintain an average discharge air
(D/A) temperature by:
1. monitoring the discharge air temperature sensor;
and
2. modulating economizer dampers and sequencing
stages of mechanical cooling “On” or “Off”, as
required.
The W7100A receives analog input from the discharge air
sensor mounted in the supply duct every 2 to 3 seconds by
pulsing DC current across the sensor, then “reading” the
voltage potential across this thermistor.
If the comparison between the setpoint and the actual dis-
charge air temperature indicates that cooling is required,
the W7100A attempts to satisfy the load by modulating the
economizer open (if applicable).
Economizer Cycle
The economizer is only allowed to function if the ambient
conditions are below the setpoint of the enthalpy switch.
If the ambient air conditions are above the enthalpy set-
point, the W7100A will open the Fresh Air dampers to the
minimum setpoint position.
To take full advantage of the “free cooling” provided by the
economizer, the W7100A “resets” the discharge air setpoint.
The amount of “reset” between the actual discharge air
setpoint and the economizer control point is equal to
1/2 of the W7100’s control band setpoint.
Example: With a typical control band setting of 6 o F, the
amount of discharge air “reset” is 3 o F (1/2 of the
control band setpoint). Therefore, if the discharge air
setpoint is 55
o
F, the economizer control point is 52
o
F
(i.e., 55 o F - 3 o F).
A second economizer “algorithm” within the W7100A is the
response time of the controller. The greater the amount of
deviation between the discharge air temperature and the
economizer control point, i.e., as the temperature strays fur-
ther from the control point, the response time becomes
faster; and, as the discharge air temperature approaches
the control point, the response time becomes slower.
When the discharge air temperature is within the “Dead-
band” (± 1.5
o F of the economizer control point); the
W7100A maintains the economizer’s present position.
When the economizer can not handle the cooling require-
ment or when the outdoor ambient conditions are unsuit-
able for “economizing”, the W7100A activates the unit’s me-
chanical cooling section.
Note: As long as ambient conditions are suitable
for economizing, the economizer works in
conjunction with the mechanical cooling operation.
The control algorithm used by the W7100A to add stages of
cooling is illustrated in Figure 5-1. When the discharge air
temperature drifts above the setpoint, “Region 1”, a stage of
mechanical cooling is added based on time and the amount
of deviation from setpoint. If the discharge air temperature
remains above the setpoint, the W7100A energizes addi-
tional stages of mechanical cooling.
If the operating cooling stage is capable of satisfying the
cooling requirement, as the discharge air temperature falls
below the setpoint for a sufficient period of time, the
W7100A turns the stages of mechanical cooling “Off”, “Re-
gion 3”.
The W7100A determines the length of the time before
stages of mechanical cooling are turned “On” and “Off”.
When the system is operating within the control band, the
delay is longest at setpoint, and decreases to a minimum of
4 minutes when the discharge air temperature exceeds the
upper or lower limit of the control band. Refer to the illustra-
tion in Figure 5-1.
Figure 5-1
W7100A Staging Sequence
System Start-Up